Digital recording of picture image data made great progress and has been put in practical use in recent years.
In the digital recording of picture image data, if images are directly digitized, too much information results and high running costs are required for recording or reproducing the signals. In order to solve this problem, a high efficiency encoding technique, such as the international MPEG (Motion Picture Expert Group) standard, is carried out.
However, when such encoded digital picture image data are recorded on a magnetic tape by a helical scanning type video tape recorder (VTR) equipped with rotary heads and the recorded data are reproduced at tape speeds differing from the recording speed, the reproducing heads run across multiple tracks of the tape as the tape feeding speed is changed and picture image data are intermittently obtained. This creates a lack of the continuity in the data. Therefore, if such encoded data are recorded on a tape, the reproduction of data at variable speeds will become extremely difficult.
In order to overcome such a defect, a proposal has been made to limit the number of reproduction speeds and arrange data on tracks that are reproducible by reproducing heads for the particular reproduction speeds.
One tape format for a conventional digital VTR is shown in FIG. 20. Data are recorded or reproduced on or from oblique tracks 2 of a magnetic tape 1 using rotary heads as shown in FIG. 21. Pilot signals F1, F0 and F2, which are required for tracking, are recorded on the tracks 2 in that particular sequence. The tracks 2 are formed so that adjacent tracks form an opposite azimuth and the pilot signals are recorded by superimposing them on picture image data and voice data. The pilot signals are recorded in repetitive cycle periods of 4 signals F0, F1, F0 and F2. Each of the tracks 2 is provided with a picture image data area and a voice data area as shown in FIG. 20. 10 tracks are used per 1/30 sec., which is equivalent to a time of one frame.
The construction of the rotary head is shown in FIG. 21. The rotary head contains a "+" azimuth head 4 and a "-" azimuth head 5. The "-" azimuth head is 180.degree. away from the head 4 on a rotary drum 3.
The construction of one GOP (group of pictures) picture image data in the MPEG standard is shown in FIG. 22. One GOP is comprised of 12 frames. Out of these frames, the I-frame contains only intra-frame encoded data, the frame P contains the intra-frame data and inter-frame predictive encoded data from the preceding I-frame or the intra-frame and inter-frame data from the preceding P-frame, and the frame B contains either the intra-frame and the preceding I-frame or the preceding P-frame and the inter-frame data from the succeeding I-frame data or the succeeding P-frame.
FIG. 23(a) shows the relationship between the track pattern and the scanning loci of the rotary heads during reproduction at +5(-3), +9(-7) and +17(-15) times speeds. The symbol "+" indicates when a tape is run in the same direction as the recording direction while the symbol "-" indicates when a tape is run in the direction reverse to the recording direction and the numerals show the number if times the tape speed is faster than the recording speed.
For simplicity of explanation, the track pattern is shown by mapping the normally oblique tracks onto tracks which are vertical with respect to the longitudinal direction of the tape, in the drawings used for the following explanations. In FIG. 23(b), envelopes of picture image data reproduced at the speeds shown in FIG. 23(a) are illustrated. The envelopes shown in FIG. 23(b) are the envelopes reproduced by the reproducing head having the "+" azimuth at the respective speeds shown in FIG. 23(a).
As shown in FIGS. 23(a) and 23(b), picture image data can be reproduced at respective ends A and B and the center C of the track 2, which are common to all the speeds described above.
Accordingly, when data areas for the particular speed reproduction are provided on a magnetic tape, they are normally provided at respective ends A and B and the center C of the track 2 for carrying out the particular speed reproduction. In the reproduction data areas A, B and C within one track, particular speed reproduction data generated by low frequency components (DC and low frequency components) of DCT (Digital Cosine Transformation) data of a frame equivalent to the upper 1/4 of the I-frame, as shown in FIG. 25, are recorded. Then, the same data are repetitively recorded over the adjacent 17 tracks. Thus, the continuity of data recorded in the reproduction data areas for particular speeds provided at the center and the respective ends of the tracks is maintained. Consequently, even when the repetitive recording causes the picture image data to be reproducible in any track, low frequency component data equivalent to the upper 1/4 of the next I-frame are recorded in the reproduction data areas of the next 17 tracks, and it becomes possible to achieve the particular speed reproduction of picture image data recorded in the reproduction data areas for particular speeds by updating the I-frame by 1/4 at the +5(-3), +9(7) and +17(15) time speeds.
By the way, a VTR which records or reproduces high efficiency encoded picture image data as described above for particular speed reproduction uses limited areas that are traceable by the reproducing heads at a low speed as well as a high speed. Furthermore, the same particular speed reproduction data are used at all speeds, creating deterioration in the video quality that is not conspicuous at a high speeds but is very noticeable at low speeds.
As described above, in a conventional digital VTR, there was a problem with the deterioration in video quality that is not conspicuous at high speeds but is noticeable at a low speed.